As electronic devices are miniaturized more and more, demand for lithium ion secondary batteries having a high voltage and a high energy density is increasing. However, it is also demanded more and more to reduce the cost of the lithium ion secondary batteries as they are now widely used. While heat generation during charging and discharging is of concern when an organic electrolyte is used, bonding states of a positive electrode and a negative electrode by sintering are important when a solid electrolyte is used. Sintering conditions (for example, temperature, atmosphere, etc.) for achieving a good bonding state are not necessarily the same for sintering the positive electrode as those for sintering the negative electrode such that some kind of tuning the conditions is preferably implemented.
For example, Japanese patent application publication No. H04-037646 discloses an art of enclosing a zirconia-based conductive ceramic film with a ceramic during a sintering process so as not to emit heat from the heater directly to the film. Also, Japanese patent application publication No. H06-009268 discloses an art of sintering a zirconia green sheet while applying a load using a weight. And Japanese patent application publication No. 2001-114577 discloses a porous ceramic thin plate to be utilized as a setter or a spacer, etc., in sintering a ceramic green sheet of a solid electrolyte material has. Furthermore, International publication WO99/055639 closes a solid electrolyte ceramic sheet manufactured by sintering a green sheet to be sintered in a state that it is sandwiched between spacers. Yet furthermore, International publication WO99/059936 discloses setters of ceramic body in a sheet form and a ceramic sheet for a solid electrolyte fuel cell which is sintered in a state that it is sandwiched by the setters on both sides thereof. Japanese patent application publication No. 2007-001860 discloses a porous ceramic thin plate to be utilized as a setter used in sintering a ceramic green sheet to be formed into a solid electrolyte material or as a spacer or the like for sintering a sheet disposed on and/or overlaid with the spacer.
However, the above conventional art merely discloses inventions which can prevent direct reaction with an original plate material by placing the green sheet to be sintered on the setter and prevent components in the green sheet from being decomposed and released from an upper surface of the green sheet by placing the setter on the upper surface such that the conventional inventions merely provide the way to confine the green sheet by enclosing it with a lid.
Here, in the case of a lithium ion battery produced through a sintering process, for example, it is generally preferable to fire an electrode material on a positive electrode side in a reducing atmosphere and to fire an electrode material on a negative electrode side in air or an oxidizing atmosphere when the electrode materials are metal oxide. Also, for example in a case where electrode materials on the positive electrode side and the negative electrode side differ significantly in the melting point, the positive electrode and the negative electrode are preferably sintered separately. Since a positive electrode green sheet and a negative electrode green sheet are thus sintered individually and these are bound to a separately-sintered electrolyte green sheet, all green sheets cannot be sintered integrally and the productivity is poor.
Further features of the present invention, its nature, and various advantages will be more apparent from the accompanying drawings and the following description of the preferred embodiment.